1. Field of the Invention
The present invention relates to an apparatus for recording and/or reproducing variable bit rate data. More specifically, the present invention relates to a recording apparatus in which variable bit rate data being processed by MPEG2, for example, is recorded in a data storage media such as an optical disc, or a reproducing apparatus in which the variable bit rate data is reproduced from the optical disc.
2. Description of the Prior Art
An MPEG is known as an international standard for compressing digital data of a moving picture. In the MPEG, in order to increase an image quality, a variable bit rate is allowed. The variable bit rate means that a bit rate Mbps! varies within a predetermined range as shown in FIG. 1, for example.
In an MPEG2, a compression degree, i.e. a bit rate is changed in accordance with a scene where deterioration of an image is not compricious even if the compression degree is made large or a scene where deterioration of an image is compricious if the compression degree is made large.
Such the variable bit rate data is recorded in a data storage media (DSM) such as an optical disc, for example. When the variable bit rate data in the DSM is reproduced, a buffer memory is utilized because there is a difference between a speed for reading the data from the DSM (hereinafter, called as "transfer rate") and a speed for decoding read data and the difference is not constant. More specifically, the variable bit rate data read from the DSM with a given transfer rate is once stored in a system buffer, and the data in the system buffer is read in response to a read request from a decoder (MPEG decoder, for example).
When a writing speed of the variable bit rate data into the system buffer, i.e. the transfer rate is larger than the reading speed from the system buffer, a control for reading the buffer memory is simple. That is, the reading of the data from the DSM may be suspended when the buffer memory is filled, and the reading of the data from the DSM may be restarted when a data amount within the buffer memory becomes less than a predetermined lower limit value. That is, in such a case, it is possible to always secure the data amount within the system buffer to be larger than a data amount necessary for the decoder.
In contrast, if the transfer rate is smaller than the reading speed, there is a possibility that the data amount within the memory becomes smaller than the data amount necessary for the decoder, and in such a case, resultingly, an operation of the decoder is performed intermittently.
Such a problem can be avoided by making the capacity of the buffer memory to be large. In a case where the capacity of the buffer memory is large, not only it takes a long time for storing data of a sufficient amount in the buffer memory at the beginning of reproducing but also a reproducing apparatus becomes large and a cost becomes high.